IK rig handle stretch
The easiest way to fix it would be to copy IK target, that you already have (It's R.Hand I presume), call it R.Hand_IK or something of that meaning. From what I see on the image it has no parent already - good.Disable Deform option for this bone (showed on the image below). Parent old R.Hand bone to arm bone (don't know how you called it, but it's the yellow one on the image). Remove IK constraint from the arm bone and add it to R.Hand bone. Then make R.Hand_IK target of this constraint.